The present invention relates to a method of preparing .alpha.,.beta.-unsaturated carboxylic acid esters by the reaction of .alpha.,.beta.-unsaturated halogen compounds with carbon monoxide and alkali alcoholates in the present of alcohols and catalysts.
Setting out from .alpha.,.beta.-unsaturated halides, known reactions are used to produce predominantly vinyl acetic acid esters or their homologs, such as .beta.,.gamma.-carboxylic acid esters, that is, those in which the position of the double bond is the same as it was in the halide, while .alpha.,.beta.-unsaturated carboxylic acid esters, in which the position of the double bond is shifted one position in the chain, form not at all or only in small amounts as by-products.
Thus it is known that crotonic acid methyl ester in small amounts plus vinyl acetic acid methyl ester are formed from allyl chloride with carbon monoxide and methanol in the presence of catalytic amounts of nickel carbonyl and thiourea at atmospheric pressure and at a temperature between 15.degree. and 35.degree. C. at a pH between 5.5 and 9. Vinyl acetic acid methyl ester, however, can be transformed to crotonic acid methyl ester only with great difficulty, for example by heating with 10 wt.-% of basic ion exchanger at 100.degree. C. for 10 hours to produce a yield of up to 70%, or by treatment with 2% methanolic NaOH at room temperature, although approximately 30% of 2-methoxybutyric acid methyl ester is produced (German "Offenlegungsschrift" No. 1,936,725).
It is furthermore known that palladium salts catalyze the carbonylation of allyl chloride at approximately 100 atmospheres of carbon monoxide pressure. In the presence of an inert solvent such as benzene, vinyl acetyl chloride forms; in an alcohol as solvent, the corresponding vinyl acetic acid esters form. Crotonic acid chloride or crotonic acid esters are produced only as by-products by the isomerization of the double bond (J. Tsuji, J. Kiji, S. Imamura and M. Morikawa, J.Amer.Chem.Soc., 86, 4350 (1964) and D. Medema, R. van Helden and C. F. Kohll, Inorg. Chim. Acta, 3, 255 (1969).
Disadvantages of both these processes lie in the fact that the preparation of the carboxylic acid esters must be performed in two steps, that only part of the vinyl acetic ester is reacted to crotonic acid esters, that in the one case the reaction times are too long, and in the other case very appreciable amounts of by-products are formed.
Another approach to the manufacture of crotonic acid esters consists in the oxidation of crotonic aldehyde to crotonic acid (Ullmann 1960, vol. 5, p. 617), followed by esterification (Jeffery and Vogel, J.Chem.Soc. 1948, 666). Here, again, two process steps are required, and the yields are about 80%.